The performances of GaN-based HEMTs as RF power amplifiers are limited by reliability issues such as current collapse. C-doping in GaN buffers reduces butter leakage and improves breakdown voltage but at the same time, introduces acceptor traps leading to current collapse. In this paper, butter trapping effects have been studied for different AlInN/GaN HEMTs in light of drain lag transients. Three different GaN butter structures have been considered. Initially, HEMT having unintentionally doped GaN butter is fabricated, and its experimental results are used for simulation validation. Then, simulation models are extended for HEMTs having C-doped buffers. In the simulations, acceptor type of traps are considered in the doped GaN butter layers. Self-heating effects are also taken into account. The drain lag turn-on mixed-mode 2D TCAD simulations are carried out to analyze the dynamic responses. An optimum butter structure is proposed based on transient results. A two-layer butter having an unintentionally doped GaN layer near the channel and C-doping in the rest shows minimum butter trapping effects. The optimum thickness of the unintentionally doped layer is estimated as 400 nm. Further simulations reveal the impact of C-doped layer thickness and trap concentrations in drain lag transients. © 2020 IEEE.